1. Field of the Invention
This invention relates to cameras having an electric motor as the drive source of an operating mechanism for the photographic lens, for example, and more particularly to improvements of the control of braking force on the motor.
2. Description of the Related Art
Recently, so-called "full-automatic" cameras have been developed wherein electric motor or like electromagnetic drive sources are generally used. Particularly with the D.C. motor type of drive sources rotation speed or torque depends on the voltage (or current) applied thereto. When the actual voltage of the electrical power source or battery varies at random, setting aside its inability to control the stationary rotation accurately, even for the stoppage of the rotating motor, the kinetic energy differs from cycle to cycle of operation of the camera, and the number of revolutions of the motor in the timer interval from the moment at which the current supply has been cut off to the moment at which the motor actually stops (or the distance the operating mechanism moves for this time interval) by means of the inertial energy varies and hence fails to control accurately. The conventional methods for reducing the inertial rotation as far as possible are to mechanically bar directly the operating mechanism, or to electrically bring both ends of the winding of the motor into short circuit contact with each other by using a metal connector or switching elements. Though these methods have very often been employed in the past, their ability to immediately stop the operating mechanism when the current supply to the motor is cut off was, in both cases, not improved beyond a certain low limit, owing to the presence of a backlash of the mechanical bar, or the insufficient braking effect of the short-circuiting.
Such a random error in the over-running distance of the operating mechanism gives rise to a problem of differing the length through which the film is advanced one frame from cycle to cycle of winding operation with the result that the spacing between the successive two of the exposed frames is not maintained constant. In application to the automatic focusing of the photographic lens, another problem arises in that the accuracy of focusing control cannot be improved. It will, therefore, be appreciated that the reduction of the random difference in the number of inertial revolutions of the motor leads to a possibility of improving the various capabilities of the full-automatic camera.
Attempts have been made to minimize the range of variation of the number of inertial revolutions of the motor by employing a constant voltage drive method, or another drive method of constant current for the stationary rotation. But, if the electrical power source is a battery, the circuit will become very complicated, and a high price will result. Therefore, this method cannot be said to be effective. Also, the advantage arising from the maintenance of the constant speed of the stationary rotation of the motor is ineffective when the magnitude of braking force by the above-described short-circuiting means is left not to vary as the function of variables such as the frictional force of the operating mechanism (for winding up the film) which varies with variation of the ambient temperature and humidity, and the resistance of the winding of the motor. To effect the aforesaid advantage it may be considered to feed the actual braking distance or the actual magnitude of braking force back to control the operation of the motor so that the number of inertial revolutions is maintained always constant. But, since the braking time or distance is generally very short, the use of the feedback technique led to a drawback in that the production cost is remarkably increased because the detector for the speed of the motor and the braking control circuit are of very complicated form.